Revisiting the Norrish type II reactions

We have been fortunate to have Niklas Heine visit our lab from the University of Münster, where he is working on a PhD degree in the laboratory of Armido Studer. Niklas has brought with him a lot of expertise in radical reactions. These sorts of visits are great for my students as they encourage us to try things we have wanted to attempt for a while, but have encountered high barriers due to lack of familiarity with certain kinds of techniques. Those of you who follow my posts know that this is a general frustration of mine – I refer to convincing students to use methods that are less common than all those round bottom flask getups. For many years we have not done any electrochemistry, which I absolutely love. My lab published a lot in that area thanks to Tung Siu, who ran anodic oxidation experiments 15 years ago. He came to us with electrical engineering background, which is why electrosynthesis was a natural fit for him. Thus, I am not really laying any blame on others who might not have viewed the use of wires with the same level of enthusiasm. I even had a company, Ylektra, who pioneered parallel electrosynthesis (Tung worked there for a while). Ylektra was the wholly owned subsidiary of Affinium Pharmaceuticals, which was in turn acquired by the Swiss company Debiopharm last year… So there is a lot of good stories ultimately rooted in electrons!

When it comes to photochemistry, there is a surprisingly curious connection with electrochemical reactions. This connection has less to do with the phobia of getting started (in fact, electrochemistry is trickier), but is more about conceptual similarity in that radical cations are often involved as intermediates in electrochemical and photochemical reactions alike. But not all the time…

The lost art of Norrish type II reactions is the scientific part of today’s post. Below is a fantastic example of its use in total synthesis by Leo Paquette. The ketone you see was taken up in dioxane and placed in a Pyrex tube. The resulting solution was deoxygenated, and then irradiated using a medium pressure Hanovia lamp for 10 h. The process was run on a 100mg scale and delivered a 62% yield (not bad!). As with many photochemical reactions, the reaction was fairly dilute (10mL of dioxane was used). Still though, this a great example of C-H activation… Isn’t it? Of course it is.


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